1. Field of Invention
The invention provides production-worthy ion sources and methods capable of using new source materials, in particular, heat-sensitive materials such as decaborane (B10H14), and hydrides and dimer-containing compounds novel to the ion implantation process, to achieve new ranges of performance in the commercial ion implantation of semiconductor wafers. The invention enables shallower, smaller and higher densities of semiconductor devices to be manufactured, particularly in Complementary Metal-Oxide Semiconductor (CMOS) manufacturing. In addition to enabling greatly enhanced operation of new ion implanter equipment in the manufacture of semiconductor devices, the invention enables the new ion source to be retrofit into the existing fleet of ion implanters with great capital cost savings. Embodiments of the invention uniquely implant decaborane and the other dopant materials in particularly pure ion beams, enabling a wide range of the needs of a fabrication facility to be met. Various novel constructional, operational and process features that contribute to the cost-effectiveness of the new technology are applicable as well to prior technology of the industry.
2. Description of the Prior Art
As is well known, ion implantation is a key technology in the manufacture of integrated circuits (ICs). In the manufacture of logic and memory ICs, ions are implanted into silicon or GaAs wafers to form transistor junctions, and to dope the well regions of the p-n junctions. By selectively controlling the energy of the ions, their implantation depth into the target wafer can be selectively controlled, allowing three-dimensional control of the dopant concentrations introduced by ion implantation. The dopant concentrations control the electrical properties of the transistors, and hence the performance of the ICs. A number of dopant feed materials have previously been used, including As, Ar, B, Be, C, Ga, Ge, In, N, P, Sb and Si. For those species which are of solid elemental form, many are obtainable in gaseous molecular form, such as fluoride compounds that are ionizable in large quantities at significantly elevated temperatures. The ion implanter is a manufacturing tool which ionizes the dopant-containing feed materials, extracts the dopant ions of interest, accelerates the dopant ions to the desired energy, filters away undesired ionic species, and then transports the dopant ions of interest to the wafer at the appropriate energy for impact upon the wafer. The presence in the implanter of certain elements, such as the disassociated element fluorine, is detrimental to the implanted wafers, but, despite such drawbacks, trace amounts of such contaminants have been tolerated in many contexts, in the interest of achieving production-worthy throughput volume. Lower contaminant levels from what is now achievable is desired.
In a complex relationship, overall, a number of variables must be controlled in order to achieve a desired implantation profile for a given ion implantation process:                The nature of the dopant feed material (e.g., BF3 gas)        Dopant ion species (e.g., B+)        Ion energy (e.g., 5 keV)        Chemical purity of the ion beam (e.g., <0.01% energetic contaminants)        Isotopic purity of the ion beam (e.g., ability to discriminate between 113In and 115In)        Energy purity of the ion beam (e.g., <2% full width at half maximum, i.e. FWHM)        Angular divergence and spatial extent of the beam on the wafer        Total dose (e.g., 1015 atoms/cm2) implanted on the wafer        Uniformity of the dose (e.g., ±1% variation in the implanted density over the total wafer surface area).        
These variables affect the electrical performance, minimum manufacturable size and maximum manufacturable density of transistors and other devices fabricated through ion implantation.
A typical commercial ion implanter is shown in schematic in FIG. 1. The ion beam I is shown propagating from the ion source 42 through a transport (i.e. “analyze”) magnet 43, where it is separated along the dispersive (lateral) plane according to the mass-to-charge ratio of the ions. A portion of the beam is focused by the magnet 43 onto a mass resolving aperture 44. The aperture size (lateral dimension) determines which mass-to-charge ratio ion passes downstream, to ultimately impact the target wafer 55, which typically may be mounted on a spinning disk 45. The smaller the mass resolving aperture 44, the higher the resolving power R of the implanter, where R=M/ΔM (M being the nominal mass-to-charge ratio of the ion and ΔM being the range of mass-to-charge ratios passed by the aperture 44). The beam current passing aperture 44 can be monitored by a moveable Faraday detector 46, whereas a portion of the beam current reaching the wafer position can be monitored by a second Faraday detector 47 located behind the disk 45. The ion source 42 is biased to high voltage and receives gas distribution and power through feedthroughs 48. The source housing 49 is kept at high vacuum by source pump 50, while the downstream portion of the implanter is likewise kept at high vacuum by chamber pump 51. The ion source 42 is electrically isolated from the source housing 49 by dielectric bushing 52. The ion beam is extracted from the ion source 42 and accelerated by an extraction electrode 53. In the simplest case (where the source housing 49, implanter magnet 43, and disk 45 are maintained at ground potential), the final electrode of the extraction electrode 53 is at ground potential and the ion source is floated to a positive voltage Va, where the beam energy E=qVa and q is the electric charge per ion. In this case, the ion beam impacts the wafer 55 with ion energy E. In other implanters, as in serial implanters, the ion beam is scanned across a wafer by an electrostatic or electromagnetic scanner, with either a mechanical scan system to move the wafer or another such electrostatic or electromagnetic scanner being employed to accomplish scanning in the orthogonal direction.
A part of the system of great importance in the technology of ion implantation is the ion source. FIG. 2 shows diagrammatically the “standard” technology for commercial ion sources, namely the “Enhanced Bernas” arc discharge ion source. This type of source is commonly the basis for design of various ion implanters, including high current, high energy, and medium current ion implanters. The ion source a is mounted to the vacuum system of the ion implanter through a mounting flange b which also accommodates vacuum feedthroughs for cooling water, thermocouples, dopant gas feed, N2 cooling gas, and power. The dopant gas feed c feeds gas, such as the fluorides of a number of the desired dopant species, into the arc chamber d in which the gas is ionized. Also provided are dual vaporizer ovens e, f inside of the mounting flange in which solid feed materials such as As, Sb2O3, and P may be vaporized. The ovens, gas feed, and cooling lines are contained within a water cooled machined aluminum block g. The water cooling limits the temperature excursion of the aluminum block g while the vaporizers, which operate between 100 C and 800 C, are active, and also counteracts radiative heating by the arc chamber d when the ion source is active. The arc chamber d is mounted to, but designedly is in poor thermal contact with, the aluminum block g. The ion source a employs an arc discharge plasma, which means that it operates by sustaining within a defined chamber volume a generally narrow continuous electric arc discharge between hot filament cathode h, residing within the arc chamber d, and the internal walls of the arc chamber d. The arc produces a narrow hot plasma comprising a cloud of primary and secondary electrons interspersed with ions of the gas that is present Since this arc can typically dissipate in excess of 300 W energy, and since the arc chamber d cools only through radiation, the arc chamber in such Bernas ion sources can reach a temperature of 800 C during operation.
The gas is introduced to arc chamber d through a low conductance passage and is ionized through electron impact with the electrons discharged between the cathode h and the arc chamber d and, as well, by the many secondary electrons produced by the arc discharge. To increase ionization efficiency, a substantial, uniform magnetic field i is established along the axis joining the cathode h and an anticathode j by externally located magnet coils, 54 as shown in FIG. 1. This provides confinement of the arc electrons, and extends the length of their paths. The anticathode j (sometimes referred to as a “repeller”) located within the arc chamber d but at the end opposite the cathode h is typically held at the same electric potential as the cathode h, and serves to reflect the arc electrons confined by the magnetic field i back toward the cathode h, from which they are repelled back again, the electrons traveling repeatedly in helical paths. The trajectory of the thus-confined electrons results in a cylindrical plasma column between the cathode h and anticathode j. The arc plasma density within the plasma column is typically high, on the order of 1012 per cubic centimeter, this enables further ionizations of the neutral and ionized components within the plasma column by charge-exchange interactions, and also allows for the production of a high current density of extracted ions. The ion source a is held at a potential above ground (i.e., above the potential of the wafer 55) equal to the accelerating voltage Va of the ion implanter: the energy, E, of the ions as they impact the wafer substrate is given by E=qVa, where q is the electric charge per ion.
The cathode h of such a conventional Bernas arc discharge ion source is typically a hot filament or an indirectly-heated cathode which thermionically emits electrons when heated by an external power supply. It and the anticathode are typically held at a voltage Vc between 60V and 150V below the potential of the ion source body Va. Once an arc discharge plasma is initiated, the plasma develops a sheath adjacent the exposed surface of the cathode h within chamber d. This sheath provides a high electric field to efficiently extract the thermionic electron current for the arc; high discharge currents (e.g., up to 7A) can be obtained by this method.
The discharge power P dissipated in the arc chamber is P=D Vc, typically hundreds of watts. In addition to the heat dissipated by the arc, the hot cathode h also transfers power to the walls of the arc chamber d. Thus, the arc chamber d provides a high temperature environment for the dopant arc plasma, which boosts ionization efficiency relative to a cold environment by increasing the gas pressure within the arc chamber d, and by preventing substantial condensation of dopant material on the hot chamber walls.
If the solid source vaporizer ovens e or f of the Bernas arc discharge ion source are used, the vaporized material feeds into the arc chamber d with substantial pressure drop through narrow vaporizer feeds k and l, and into plenums m and n. The plenums serve to diffuse the vaporized material into the arc chamber d, and are at about the same temperature as the arc chamber d. Radiative thermal loading of the vaporizers by the arc chamber also typically prevents the vaporizers from providing a stable temperature environment for the solid feed materials contained therein below about 200 C. Thus, typically, only solid dopant feed materials that both vaporize at temperatures >200 C and decompose at temperatures >800 C (the temperature of the walls of the ionization chamber of a typical Bernas source) can be successfully vaporized and introduced by this method.
A very significant problem which currently exists in the ion implantation of semiconductors is the limitation of production-worthy ion implantation implanters that prevents effective implanting of dopant species at low (e.g., sub-keV) energies at commercially desired rates. One critically important application which utilizes low-energy dopant beams is the formation of shallow transistor junctions in CMOS manufacturing. As transistors shrink in size to accommodate more transistors per IC according to a vital trend, the transistors must be formed closer to the surface of the target wafer. This requires reducing the velocity, and hence the energy, of the implanted ions, so that they deposit at the desired shallow level. The most critical need in this regard is the implantation of low-energy boron, a p-type dopant, into silicon wafers. Since boron atoms have low mass, at a given energy for which the implanter is designed to operate, they must have higher velocity and will penetrate deeper into the target wafer than other p-type dopants; therefore there is a need for boron to be implanted at lower energies than other species.
Ion implanters are relatively inefficient at transporting low-energy ion beams due to space charge within the ion beam, the lower the energy, the greater the problem. The space charge in low energy beams causes the beam cross-section area (i.e. its “profile”) to grow larger as the ions proceed along the beam line (there is “beam blow-up”). When the beam profile exceeds the profile for which the implanter's transport optics have been designed, beam loss through vignetting occurs. For example, at 500 eV transport energy, many ion implanters currently in use cannot transport enough boron beam current to be commercially efficient in manufacturing; i.e., the wafer throughput is too low because of low implantation dose rate. In addition, known ion sources rely on the application of a strong magnetic field in the source region. Since this magnetic field also exists to some extent in the beam extraction region of the implanter, it tends to deflect such a low-energy beam and substantially degrade the emittance properties of the beam, which further can reduce beam transmission through the implanter.
An approach has been proposed to solve the problem of low-energy boron implantation: molecular beam ion implantation. Instead of implanting an ion current I of atomic B+ ions at an energy E, a decaborane molecular ion, B10Hx+, is implanted at an energy 10×E and an ion current of 0.10×I. The resulting implantation depth and dopant concentration (dose) of the two methods have been shown to be generally equivalent, with the decaborane implantation technique, however, having significant potential advantages. Since the transport energy of the decaborane ion is ten times that of the dose-equivalent boron ion, and the ion current is one-tenth that of the boron current, the space charge forces responsible for beam blowup and the resulting beam loss can potentially be much reduced relative to monatomic boron implantation.
While BF3 gas can be used by conventional ion sources to generate B+ ions, decaborane (B10H14) must be used to generate the decaborane ion B10Hx+. Decaborane is a solid material which has a significant vapor pressure, on the order of 1 Torr at 20 C, melts at 100 C, and decomposes at 350 C. To be vaporized through preferred sublimination, it must therefore be vaporized below 100 C, and it must operate in a production-worthy ion source whose local environment (walls of the ionization chamber and components contained within the chamber) is below 350 C to avoid decomposition. In addition, since the B10H14 molecule is so large, it can easily disassociate (fragment) into smaller components, such as elemental boron or diborane (B2H6), when subject to charge-exchange interactions within an arc discharge plasma, hence it is recognized that conventionally operated Bernas arc plasma sources can not be employed in commercial production, and that ionization should be obtained primarily by impact of primary electrons. Also, the vaporizers of current ion sources cannot operate reliably at the low temperatures required for decaborane, due to radiative heating from the hot ion source to the vaporizer that causes thermal instability of the molecules. The vaporizer feed lines k, l can easily become clogged with boron deposits from decomposed vapor as the decaborane vapor interacts with their hot surfaces. Hence, the present production-worthy implanter ion sources are incompatible with decaborane ion implantation. Prior efforts to provide a specialized decaborane ion source have not met the many requirements of production-worthy usage.
More broadly, there are numerous ways in which technology that has been common to the industry has had room for improvement. Cost-effective features, presented here as useful in implementing the new technology, are applicable to implementation of the established technology as well.